While trying to understand DNA supercoiling, I came across these lines in the book Genetics by Ursula Goodenough :

All natural DNA is superhelical. The axis of the duplex itself follows
a helical path in space. For virtually all DNA, the sense of the
superhelical turns is opposite to that of duplex, thereofer, they are
negative supertwists.

1 Answer
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It means that the direction in which the normal double-strand is wound up (which is also helical) and the direction on which the double helix itself is twited to reach the supercoils are opposite to each other. Have a look at this image from the Wikipedia, I think it makes this clearer:

The first show a DNA ring with the normal helix-coiling. The second show the supercoiling of the coiled ring molecule. Imagine a rubber band. This you can twist around its band axis to get the coiling and around its vertical axis to get the supercoiling.